1. Field of Invention
The present invention relates to acoustic transducers. More specifically the present invention relates to a liquid-filled acoustic transducer which transmits acoustic energy generated within an enclosed liquid medium, through a transducer housing member, and into an exterior work medium.
2. Description of the Prior Art
The present invention is a unique acoustic energy transducing device which may be advantageously used in a preferred embodiment of the invention to modify the texture or character (i.e. the "finish") of a surface of a work material. For illustrative purposes, the following disclosure describes an application of the preferred embodiment of the invention wherein acoustic energy generated within the tool is introduced into a plastic concrete mass in order to advantageously finish an exposed surface of the plastic concrete mass. It will be understood, however, that similar tools constructed in accordance with the present invention may be used to modify the texture or character of exposed surfaces of many other materials, particularly plastic or wet materials such as plaster, wet soil, cement and the like. It will also become apparent from the following disclosure that similar tools constructed in accordance with the present invention may be used in many applications where it is advantageous or desirable to efficiently pass acoustic waves into work media for purposes other than modifying the character of a surface of the work material. For example, the present invention may be used in SONAR applications or in ultrasonic imaging in order to efficiently transmit and/or receive acoustic signals into or from adjacent work media.
When concrete is initially laid, it must be worked while it is wet in order to provide a smooth, homogeneous mixture. Working the plastic concrete helps settle the concrete and helps to densify and compact the concrete. This working of the plastic concrete also removes air voids and brings excess water and fine aggregates to the surface for subsequent finishing. After the initial finishing stages are complete more detailed work frequently commences, generally by means of hand-held floats, for purposes including the driving of suspended gravel downwards, and developing a wetted surface slurry or soup-like finish, while further driving out air pockets and the like for preparing the surface for final finishing. Thereafter, when the surface slurry is thus formed, it is conventional to employ smoothing or specialty tools (such as edgers and trowels) to provide finishing touches to the work.
It is generally known that, if not worked, the surface of wet concrete would take on a highly undesirable rough and uneven finish which, after partial setting of the concrete, would render the surface difficult, if not impossible, to finish to the desired smooth and even consistency. For this reason, it has long been known in the art that in the act of providing a first, general compacting, tamping, screeding or other such operation following the laying or dumping of the mix, various large vibrating devices may be beneficially employed. These devices generally include a rather large flat base plate, a heavy and bulky vibratory mechanism disposed thereto for moving the large plate across the concrete surface. Such devices are generally intended to provide a general smoothing and compacting operation over a large area.
When the worker has progressed to the aforementioned finishing stage wherein it is desired to provide a highly smooth surface finish, a variety of prior vibrating hand tools may be employed with varying degrees of success. Whereas such tools are, in contrast to the aforementioned larger devices, intended for hand-held operation, they retain several characteristics of the larger apparatus such as being of a rather awkward, large and heavy construction. Whereas such features may in fact be beneficial with respect to the larger devices, in a hand-held tool this bulk, weight and complexity may render the tool totally impractical for use, particularly in view of the fact that the operator is typically working for long periods of time on his knees and often in awkward positions. It must be recognized that these tools are conventionally used primarily in finishing operations wherein a great deal of vibratory energy is not required inasmuch as a mere final smoothing of the surface slurry is being effected. In these instances, a much less bulky vibrating means might be provided although, as discussed above, most designs nevertheless continue to suffer from undue weight, bulk and the like. Notwithstanding, a variety of such vibrating means have been attempted to be employed including plunger-type vibrators, air driven turbine vibrators, and even sonic air-driven orbiting-mass type vibrators.
Another type of prior vibrating hand tool is disclosed in U.S. Pat. No. 5,234,283 to Adkins. In this tool the vibratory mechanism is mounted inside the handle. The vibratory mechanism vibrates a rigid metal blade of relatively large mass by "pushing off" of the handle in an oscillating fashion. An inherent consequence of this construction is that the handle vibrates as much or more than the blade of the tool which contacts the wet concrete. These vibrations cause discomfort and difficulty of use for the operator. As a means of reducing the amount of uncomfortable vibrations transmitted through the handle to the operator, the Adkins device, in practice, is typically manufactured such that the handle/vibratory mechanism is of relatively high mass. As discussed above with respect to other prior vibrating finishing tools, it is undesirable for such tools to be heavy and bulky. Also, because only one vibrating mechanism (i.e. located in the handle and attached to the blade of the tool at one point) is typically used to drive the entire blade in the Adkins device, its blade must be constructed of particularly rigid (and therefore typically heavy and thick) material in order to cause the entire blade to vibrate in phase. A more desirable hand tool would incorporate characteristics that would cause the majority of the vibratory energy to be transmitted to the work concrete through the bottom of the device in an efficient and uniform manner and would not cause significant amounts of vibratory energy to be transmitted to the operator through the handle.
Prior patents in this area are relatively silent regarding determination of the frequency at which the vibrations should be applied to work the concrete. Because little attention is given in the prior art to the importance of determination of the frequency at which vibrations should be applied to the work material, prior vibrating concrete finishing tools typically are not provided with means by which tools' vibration frequencies can be readily changed by the user. Consequently, many prior devices do not vibrate the concrete very efficiently. Most prior concrete finishing vibrating hand tools are operated simply by turning a switch having only two settings: "on" and "off". However, in practice each batch of concrete delivered to a job site is different from the next, and a different frequency of vibration may be required from one batch to another in order to cause the desired slurry to be formed more quickly and more efficiently. This is because the natural frequency of each batch of concrete may be different from the next due to the amount of water, cement and aggregate mix that make up each particular batch. [In this context a "natural frequency" of a concrete mass is a frequency at which a standing vibration wave can be established within the concrete mass.] Accordingly, it would be desirable to provide a vibrating hand tool with multiple frequency settings, at least one of which frequencies corresponds to a natural frequency of the concrete.
Another problem with the prior art relates to the bulk of the device, per se, as well as the bulk of the power supply required to power the device. Some of the prior vibrating hand tools (such as the one disclosed in the patent to Adkins) consist of an electric cord running from the tool to a bulky battery pack which is mounted in a belt and placed around the operator's waist. Because of the inherently low electrical-to-mechanical power conversion efficiency of such devices, and because of the low vibrational energy transmission efficiency between the actuator and adjacent work media of prior devices, it is typically necessary to provide such prior devices with large power supplies. Thus, prior vibrating concrete finishing tools are typically provided with battery packs which are large and heavy, and, if worn on a waist belt, uncomfortable to the operator. In addition, in such prior devices the length of the power cord is such that it may be inadvertently dragged through the concrete. A hand tool with either a battery pack in the handle, or a less bulky hip-pack with a shorter cord would therefore be more desirable.
Common prior hand trowels typically consist of a handle and a flat metal plate which serves as a concrete-engaging blade. The trowel is used to smooth the top layer of poured concrete, but has little effect on air or water below the surface of the concrete. Conventional hand trowels are also hard to use near walls and corners because they must be wiped back and forth over the surface of the plastic concrete, and adjacent walls often present obstructions to such trowelling operations. Conventional hand trowels are also difficult to use for long periods of time because their use is physically demanding due to the high amount of friction between the blade of the tool and the concrete.
Some prior concrete finishing tool comprise gasoline powered vibrating components. Gasoline powered vibrating finishing tools cause noise pollution, harmful exhaust emissions, and do not produce a high enough output frequency to effectively vibrate the medium. They also are hard to control and virtually impossible to use in closed quarters or at edges and corners, because they are large and bulky and do not operate well adjacent to protruding vertical structures (i.e. walls and columns).
Virtually all prior concrete finishing vibratory hand tools are difficult to operate because the entire tool typically vibrates, causing difficulty for the operator.
In the present invention many of the described disadvantages of prior concrete finishing tools are overcome by introducing acoustic energy (rather than surface vibrational waves) into the concrete mass. Although no prior concrete finishing tool involves the controlled introduction of acoustic energy into a concrete work mass, acoustic transducers are known in other arts. However, prior acoustic transducers are typically of limited energy transmission efficiency due to energy transmission losses at the interfaces of adjacent media (e.g. between various internal components of the transducer itself, and between the transducer and the external medium into which the acoustic energy is intended to be transmitted).
It is known that the most efficient transmission of acoustic energy from one medium to an adjacent medium occurs when the acoustic impedances of the two media are matched. When the acoustic impedance of adjacent media are dissimilar, acoustic waves propagating from the first medium toward the second encounter a change in impedance at the interface of the two media, causing at least partial reflection of the acoustic energy back into the first medium. Thus, in order to efficiently transmit sound energy from one medium to an adjacent medium, it is desirable for the acoustic impedances of the two media to be the same at their common interface. A problem arises, however, in transmitting acoustic energy from a first medium, through a second (intermediate) medium and into a third medium, whenever the first and third media have different acoustic impedances. It will be understood that in such instances it is not possible to select an intermediate medium having a uniform acoustic impedance which matches the acoustic impedances of both the first medium and the third medium. To allow effective transmission of the acoustic waves from the first medium to the third, it is desirable for the intermediate (second) medium to have an impedance which changes in the direction of the sound propagation to match that of the first medium and the third medium at their respective interfaces with the intermediate (second) medium.
Illustrative embodiments of patents which have attempted impedance matching are U.S. Pat. Nos. 5,511,296 to Dias et al, 5,423,319 to Bolorforosh, 4,348,904 to Bautista and 5,552,004 to Lorraine et al. A problem with prior impedance matching schemes is that a match is typically made between the first medium and the second medium, or the second medium and the third medium, but never between all three media in a manner that avoids significant and abrupt impedance changes at the interfacing surfaces of the three media, or without introducing additional unmatched acoustic interfacing surfaces. An attempt at matching the acoustic impedance of three adjacent media was disclosed in the patent to Bolorforosh. In the Bolorforosh patent, a method for matching the impedance of the surface of an intermediate medium to that of another medium, preferably human tissue, was disclosed. A problem is that the impedance of the fluid (first medium) in which the sound begins propagating in the Bolorforosh device is not matched to that of the first surface encountered by the sound waves in the intermediate medium. Therefore, in the Bolorforosh device some of the waves will be reflected at this interface due to the impedance difference of the adjacent media.
In an alternative embodiment disclosed in the Bolorforosh patent, the acoustic impedance of both the leading and trailing surfaces of the intermediate medium is partially "matched" to their respective adjacent media by determining an average impedance between each surface and the adjacent medium. This is done by taking the acoustic impedance of the intermediate medium (the housing), and the acoustic impedance of the fluid (the first medium), and averaging them, thus creating an average impedance "matching" layer. The same is done between the housing and the third medium. However, this creates an impedance matching problem at four interfaces instead of two. The sound has to travel from the fluid into the first average impedance "matching" layer, from the "matching" layer into the housing, from the housing to the second "matching" layer, and from the second "matching" layer into the third medium. The impedance change at each interface is reduced by the inclusion of the average impedance "matching" layers, however, to make it to the third medium without being reflected, the acoustic waves must overcome four separate changes of acoustic impedance instead of only two.
When an acoustic wave encounters a material with a different acoustic impedance, the majority of sound that is reflected is done so at the surface of the new material. Typically, relatively little acoustic energy may be dissipated within the material compared with that which is simply reflected at the surface of the material. Therefore, the more abrupt a change in impedance there is from one material to another in the direction of sound travel, the more sound may be reflected at the interface of the two material surfaces. It therefore would be more desirable to provide an intermediate medium which has a continuous acoustic impedance gradient from one surface to the other, without any abrupt impedance changes, wherein the acoustic impedance of the leading surface matches the respective acoustic impedance of the adjacent medium with which the surface is in contact. It would be desirable for this intermediate medium to be substantially "acoustically transparent". In other words, the sound waves could pass through the substantially "acoustically transparent" medium without encountering any surfaces with abrupt acoustic impedance changes. Therefore, no acoustic waves would be reflected, and maximum efficiency of acoustic energy transfer would be achieved.
In the medical imaging field, an acoustically "transparent" medium is very desirable. In a typical imaging device, sound waves are transmitted into the body. As the acoustic waves encounter structures within the body, some of the waves are reflected due to an impedance change. The reflected waves are then sensed by a transducer, and an image is extrapolated. The higher the quality of acoustic energy transmission and reception, the higher the resolution of the generated visual image. Accordingly, a tool which can maximize the efficiency of acoustic wave transmission into a work medium, and can maximize the efficiency of acoustic waver reception from a work medium, is highly desirable.
A further problem with prior impedance "matching" schemes is that they typically require intricate manufacturing processes, such as adhesively bonding layers of dissimilar materials, or etching, cutting or molding of "microgrooves" into the materials. It is desirable to provide a method of manufacturing an acoustic impedance matching material which does not involve intricate manufacturing processes.